It is difficult to measure air data (e.g., wind speed, Angle of Attack (AOA), and Angle of Sideslip (AOS)) on a rotary aircraft, such as a helicopter. This difficulty is due in part to the air speed of rotary aircraft. The air speed of the rotary aircraft can be much slower than a typical fixed-wing aircraft. In fact, rotary aircraft have the ability to hover allowing the air speed of a rotary aircraft to approach zero. These low air speeds make it difficult to measure air data as some air data sensors are not sensitive enough at low speeds to obtain reliable air data.
Air data sensors typically use pressure sensors to make air data measurements. Pressure sensors may be used to measure air flow as pressure is proportional to velocity. However, some air data sensors use temperature sensors to measure air flow using the theory of heat transfer.
Another difficulty in measuring air data is a result of rotor downwash, which may be described as the air flow disruptions caused by a main rotor (the rotary aircraft's rotating wing assembly) of a rotary aircraft. Rotor downwash makes obtaining cross-wind components of the air flow difficult. To avoid the rotor downwash problem, air data sensors have been placed to avoid rotor downwash. For example, air data sensors have been attached to rotor blades on the main rotor. However, this type air data system requires a rotor angle encoder, which adds complexity to the system. Further, air data sensors that have been attached to the rotor blades are placed in a harsh environment, which may cause reliability problems.
Air data sensors have also been placed in chambers to overcome the problems associated with rotor downwash. However, once the air flow enters the chamber it is difficult to separate the air flow into distinct x, y, z components (i.e., air flow traveling in different directions). As a result, it is difficult to measure the speed and direction of the cross-wind components of the air flow using this type of air data system.
Accordingly, it would be beneficial to have an air data system and method that can reliably measure air data, including the cross-wind components of the air flow, for rotary aircraft.